Development of electric vehicles as substitutes for gasoline vehicles is one of important measures for global environmental issues. For this, studies have been conducted for the use of energy storage elements such as non-aqueous electrolyte secondary batteries as a power supply for the electric vehicles. Here, improvement in battery performance such as high temperature preservation properties is essential to efficient use of the energy storage element.
For this reason, the related art has proposed an energy storage element using a non-aqueous electrolyte to which an ionic metal complex is added (for example, see Patent Literature 1). Such an energy storage element can improve battery performance such as high temperature preservation properties by using the non-aqueous electrolyte prepared by adding the ionic metal complex represented by a formula disclosed in Patent Literature 1.